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Complete Circuit: The Components That Are The Cause Of Most Starter Failures

By Mohammad Samii

If you had to categorize the components that are the cause of most starter failures, the starter solenoid, without a doubt would be on top of the list. The starter drive may come in a close second.
Some large rebuilders may opt to rebuild their starter solenoids in-house or have them built to their specifications, thus having a better handle on quality and cost. Smaller rebuilders and auto-electric shops, considering the economics and profitability, by and large choose to buy their solenoids from manufacturers or other distributors. They obviously shop around for the best price, and at times, they get a product which may or may not perform to their expectations.
In order to find out why starter solenoids go bad or stick, let's briefly review this component.
A starter solenoid that is used in automotive applications must accomplish these TWO functions and in this order:
1) Work as a solenoid, which by definition, is a device operated by electrical energy to accomplish a mechanical motion, i.e. pulling the starter drive into the flywheel.
2) After the above function is done and the drive is engaged into the flywheel, the solenoid should work as a high output relay and apply the battery power to the starter motor.
It is also understood that the solenoid should be designed and constructed so that a withdrawal from the flywheel will be accomplished as soon as the starter switch is moved away from the "START" to the "RUN" position. If this is not done properly, we get a phenomenon known as STICKING solenoids which will eventually destroy the starter and damage the flywheel.
To accomplish the above functions, there are two windings used inside the solenoid named PULL-IN and HOLD-IN coils.
The PULL-IN coil has less turn but uses heavier gauge wires where as the HOLD-IN coil has more turns but smaller diameter wires.
The operation of the starter and the relationship between these two coils can be explained in these three stages:
1) When the ignition/start switch is turned to the "start" position, the current from the positive side of the battery, through switch contacts, will get to the "S" post of the starter. At this time the two coils are in parallel, meaning the current has two paths to go through. It will go from the "S" post, through the PULL-IN coil, through the field winding and armature to ground via the negative brushes. The current will also go through the HOLD-IN windings to the ground. The combined electromagnetic force generated by flow of the current through these coils that are complementary to each other at this time, will be so strong that the solenoid plunger will be pulled in and the drive will engage the flywheel. See Figure 1 on page 40.
2) At the last stages of the plunger's travel, the contact disc will bridge the "BAT" to the "MOTOR" terminal of the solenoid and the starter motor will begin to rotate the flywheel. At the same time, this action of the disc will apply battery voltage to the other end of the PULL-IN coil that was going to the starter motor. Since now both ends of the PULL-IN coil are receiving battery voltage, the current will no longer be flowing in this coil. The plunger is held in position by the magnetic forces of the HOLD-IN coil only - thus the name. See Figure 2 on page 40.
3) After the engine starts, the ignition/start switch is released from the "START" position and there will be no current at the "S" terminal of the solenoid. The starter motor, which is still turning, will turn somewhat into a generator, inducing a current back into the field coil because of an existing CEMF (counter electromotive force), a rotating armature and motor principles. It's more or less the same as motor/generators in that when you apply power it's a motor and when turned fast enough, it is a generator. This reverse current will feed back into the solenoid coils. At this time the coils are in SERIES, but only the direction of current in PULL-IN coil is reversed! See Figure 3.
The electromagnetic forces generated at this time are opposite to each other. If the coils were made magnetically balanced, meaning with the correct amount of turns and wire size, these two magnetic forces would be equal and cancel each other out. Then the solenoid spring is able to return the plunger back, stopping the battery supply and releasing the drive from the flywheel.
If the coils are not balanced, the strength of the solenoid spring is not enough to pull the plunger back and that's when we encounter a starter that will not disengage. It is very hard to detect unbalanced solenoid coils during the free run or even during a load test. Short of using specially designed solenoid testers that can electronically check the coils for balance, you can do this very simple test that does not require any special tools.
Check the coils for continuity and amperage draw. Normally you will see twice as much amperage draw in PULL-IN coil compared to HOLD-IN coil. You can find these specs in service manuals or make your own figures from the OE units. If the coils are okay and they do not smell of burned varnish, hold the solenoid upright, put the appropriate plunger in the solenoid, and with the help of your little finger prevent it from coming off. Apply power to the "MOTOR" terminal and ground the case (or the negative post in the case of a 40-MT type solenoid). The meter (if used) will show a few amps draw, but at the same time the plunger should move in or out of the solenoid with no resistance. (The plunger will fall off the solenoid if you let your little finger go). If you feel a little drag on the plunger, the chances are good that the coils are not balanced and may stick.
Obviously this procedure cannot work on solenoids that have the contacts attached to the plunger assembly such as early non-crimp cap Ford PMGR and 28-MT, Bosch (See Figures 4 and 5 on page 41) and others.
There are also solenoid testers available that can thoroughly check for balance and performance.
It should also be noted that the proper operation of the starter solenoid in most Delco passenger car and intermediate applications depends on the integrity of the stop collar, retaining ring and thrust washer. If these parts are missing or broken, the starter drive will travel further into the flywheel, causing the solenoid plunger to go further back into the solenoid. After the ignition/start switch releases, the plunger may have gone too far back, passing a magnetic (center) point, not allowing the normal release from the flywheel, even if the coils are well balanced. This can be easily verified by looking into the core samples that have stayed engaged.
Another type of problem with solenoids is a no-crank after a hot engine that has been shut down and then tried to restart again. (A hot-soak cycle). If the size of the wires and the number of turns of PULL-IN coil windings have been compromised for the sake of economy (such as using less wire or aluminum windings), then there are more chances of a no-crank during a hot engine start.
The heat of the engine compartment increases the resistance value of the coils as well the wire leading to the "S" post. This increase in resistance will limit the current flow, thus reducing the magnetic forces generated to pull the plunger in. Normally, as soon as the engine cools off the starter solenoid will function properly.
A side note here: To overcome the no-crank condition if the problem persists, you can install and wire up an auxiliary relay such as a Ford type starter relay (Figure 4) or a Bosch relay (Figure 5) by having it installed near the starter.
The hot-soak voltage drop will have minimal effect on the operation of the relay and enough power to the "S" terminal will be supplied regardless of the temperature. The auxiliary relays have been used extensively on RVs and applications where there is a long distance between the ignition/start switch and the starter.
We have installed many such relays on turbo-charged engines where the heat of the engine compartment is a great factor. It can also be used to overcome an occasional CLICKING problem due to installation of an alarm or a bad ignition switch where there is not enough current available to pull the plunger in. (Excessive voltage drop in key-start circuit).
And finally, if the starter has been clicking but not turning (due to a worn-out solenoid contact or bad connections at the BAT terminal), the chances of an ignition switch failure will increase dramatically. If the solenoid clicks but the starter does not turn, the sum of the currents of both coils will be passing through the ignition switch contacts. This current which may exceed 30 to 35 amps, will burn the switch contacts where in a normal application, this high current will be limited to a split second and reduced immediately to less than 10 amps that is flowing through the HOLD-IN coil only, depending on the type of solenoid and applications.
The cost of a quality solenoid must be balanced against the cost of warranty returns and claims. But the investment in properly performing replacement parts is usually worth the expense up front.